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<br />• extent of approximately 7,225 square miles and includes parts of Moffat, Rio Blanco, <br />Garfield, Mesa, Delta, Pitkin, and Gunnison Counties. The Piceance Creek Basin is <br />bordered by the White River Uplift to the east, the West Elk Mountains to the <br />southeast and south, the Uncompahgre Uplift to the southwest, the Douglas Creek <br />Arch to the west (Douglas Pass), and the Axial Basin Anticline (Danforth Hills) to <br />the north (Newman 1980) (Figure 7-1). <br />Deposition of sediments into this region began with downwarping of the basin floor <br />in the Cretaceous and continued through Eocene time. Low stream gradients and <br />moderate uplift of the marginal mountains prevented significant erosion of the <br />basin's perimeter. This sequence of events resulted in the creation of the Wasatch, <br />Green River, and Uinta Formations in and around a series of landlocked lakes <br />(Bradley 1964, Tweto 1980). The sedimentary rocks from this depositional sequence <br />that are currently exposed on the surface are shown in Figure 7-1. <br />The Piceance Creek Basin is well known for its vast reserves of oil shale, as well as <br />other mineral deposits such as nahcolite (NaHCO,) and dawsonite <br />(NaAI(CO,)(OH)Z). Because of these extensive oil shale and mineral deposits, <br />abundant geologic and hydrogeologic data have been collected over the years. <br />Reserves of the saline minerals nahcolite and dawsonite within the basin have been <br />estimated at 29 and 27 billion tons, respectively (Dyni 1981). The distribution of the <br />saline minerals within the Piceance Creek Basin shows a concentric zonal order that <br />is probably related to the relative solubilities of the minerals. Dawsonite has the <br />largest areal distribution, covering most of the northern part of the basin, while <br />nahcolite occurs more to the interior, and halite is restricted to the north-central <br />part. <br />7.2.1.1 Stratigraphy <br />r~ <br />u <br />The Piceance Creek Basin contains stratified sediments ranging in age from <br />Cambrian through middle Tertiary. The northern half of the basin is deepest and <br />has the thickest stratigraphic sequence of sediments. Stratigraphically there are <br />approximately 28,000 feet of vertical sequence between the highest point on the <br />White River Uplift to the east and the lowest depth of the basin, the Precambrian <br />crystalline basement. The Precambrian crystalline basement rock is estimated to be <br />24,000 feet below ground surface in the central portion of the northern Piceance <br />Creek Sasin (Murray and Haun 1974). <br />In general, a thin veneer of unconsolidated Quaternary alluvium, valley fill, and <br />terrace deposits occupies low-lying areas once occupied by ancient river channels. <br />Below these unconsolidated sediments lie approximately 8,000 feet of Tertiary <br />sedimentary deposits. The Tertiary section consists of three major formations, the <br />Uinta (Eocene), Green River, and Wasatch (Paleocene-Eocene) Formations'. These <br />sediments grade into the unconformable top of the Cretaceous Mesa Verde Group <br />The Fort Union Formation is now considered to be part of the Wasatch Formation. <br />Amencan Soda, L.L.P. <br />Commercial Mine Plan <br />August 18, 1998 <br />7-3 <br />